I've also played with some multi-sub setups

It seems to me that the big shortfall is that it's effectiveness is inversely proportional to the effectiveness of the rooms LF treatment. Ultimately in free space, subs that vary in distance (or time of arrival) to the listening position will interfere with each others response.

So the downside is any treatment you do add to the room will actually tend to make the multi-sub less effective.

If you look at Dr Geddes approach, he is adamant that you DON'T have any ER treatment on the sidewalls. Instead the constant directivity speakers are used to eliminate L speaker to Left side reflections- but then the downside is that left speaker to right wall reflection is effectively the ISD termination (which is going to be less than ~20ms in most rooms). There was a thread where he reluctantly admitted to having some sort of cloud/kicker on the ceiling though.

Also Geddes advocates only one full range sub, and then the filler subs to be bandpass enclosures that are custom built to cover the Schroeder range (and not below)- I guess to minimize the need for time correction. If you look at his (*ahem* aggressive) posts over at DIYaudio he certainly does not approve of most multi-sub implementations.

IMO Dr Geddes overall approach is an amazing balance of available compromises- but I'm not sure it takes into account the idea of a bespoke listening room. I heard his setup at RMAF a while back- it was frankly stunning for such a humble setup. Totally the wrong market for his message though...